Zeolites are crystalline aluminosilicates having angstrom-scale pores and channels in their crystal lattice. Because sites around aluminum in the framework of aluminosilicate bear negative charges, cations for charge balancing are present in the pores, and the remaining space in the pores is usually filled with water molecules. The structure, shape and size of the three-dimensional pores in zeolites vary depending on the type of zeolite, but the diameter of the pores usually corresponds to the molecular size. Thus, a zeolite is also called “molecular sieve”, because it has size selectivity or shape selectivity for molecules entering the pores depending on the type of zeolite.
Meanwhile, zeotype molecular sieves are known in which silicon (Si) and aluminum (Al) atoms constituting the framework structure of zeolite are partially or wholly replaced by various other elements. Examples of known zeotype molecular sieves include porous silicalite-based molecular sieves free of aluminum, AlPO4-based molecular sieves in which silicon is replaced by phosphorus (P), and other zeotype molecular sieves obtained by substituting a portion of the framework of such zeolite and zeotype molecular sieves with various metal atoms such as Ti, Mn, Co, Fe and Zn. These zeotype molecular sieves are materials derived from zeolites, and do not belong to the zeolite group based on the mineralogical classification, but are commonly called zeolites in the art.
Accordingly, the term “zeolite” as used herein is meant to include the above-mentioned zeotype molecular sieves in a broad sense.
Meanwhile, a zeolite having an MFI structure is one of the most actively used zeolites and include the following types:
1) ZSM-5: MFI zeolite in which silicon and aluminum are present in a specific ratio;
2) Silicalite-1: zeolite composed only of silica; and
3) TS-1: MFI zeolite in which aluminum sites are partially replaced by titanium (Ti).
The structure of an MFI zeolite is shown in FIGS. 1A and 1B. In the MFI zeolite, approximately elliptical pores (0.51 nm×0.55 nm) are connected in a zigzag configuration to form channels extending in the a-axis direction, and approximately circular pores (0.54 nm×0.56 nm) linearly extend in the b-axis direction to form linear channels. No channels remain open in the c-axis direction.
Another zeolite, beta (BEA), adopts a truncated bipyramidal shape, with 6.6×6.7 Å channels running straight along the a- (or b-) axis and 5.6×5.6 Å channels running tortuously along the c-axis (FIG. 1C).
Powdery MFI zeolites are very widely used in household and industrial applications, including petroleum cracking catalysts, adsorbents, dehydrating agents, ion exchangers, gas purifiers, etc., meanwhile MFI zeolite thin films formed on porous substrates, such as porous alumina, are widely used as membranes for separating molecules through which molecules can be separated on the basis of size. Furthermore, MFI zeolite thin films can be used in a wide range of applications, including second- and third-order nonlinear optical thin films, three-dimensional memory materials, solar energy storage devices, electrode auxiliary materials, carriers of semiconductor quantum dots and quantum wires, molecular circuits, photosensitive devices, luminescent materials, low dielectric constant (k) thin films, anti-rusting coatings, etc.
As described above, the pore shape, size and channel structure of MFI zeolites vary depending on the crystal direction.
Meanwhile, methods for producing MFI zeolite thin films on substrates such as glass plates are broadly divided into a primary growth method and a secondary growth method. According to the primary growth method, a substrate is soaked in a gel for the synthesis of MFI zeolite without any pretreatment, and then spontaneous growth of an MFI zeolite film on the substrate is induced. The synthetic gel used herein usually contains tetrapropylammonium hydroxide (TPAOH). In this case, b-axis-oriented MFI zeolite crystals grow perpendicular to the glass substrate at the initial stage of the reaction. However, a-axis oriented crystals begin to grow subordinately from central portions of most of the crystals grown on the glass plate. In addition, with the passage of time, the crystals grow in various directions, and as a result, the final thin film has various orientations. The randomly-oriented MFI zeolite thin film is useful in some applications, but its applicability is limited. Particularly, when the randomly oriented MFI zeolite thin film is applied as a membrane for the separation of molecules, the molecular permeability, which is one of the most important factors in molecular separation, is markedly reduced. When organic bases other than TPAOH are used in the primary growth method, no MFI zeolite thin film grows on the substrate. To overcome such problems, the secondary growth method is used.
In the secondary growth method, a substrate having MFI zeolite crystals attached thereto is soaked in an MFI zeolite synthetic gel, and then allowed to react to form an MFI zeolite thin film. Herein, the MFI zeolite crystals attached to the substrate act as seed crystals. The orientation of the MFI zeolite crystals attached to the substrate plays a very important role in determining the orientation of the MFI zeolite thin film to be produced later. For example, when the a-axes of the MFI zeolite seed crystals are oriented normal to the substrate, the a-axes of the MFI zeolite thin film formed therefrom tend to be oriented normal to the substrate, and when the b-axes of the MFI zeolite seed crystals are oriented normal to the substrate, the b-axes of the MFI zeolite thin film formed therefrom tend to be oriented normal to the substrate.
However, the orientation of the resulting zeolite thin film is changed more sensitively to an organic base contained in the synthetic gel added to form the thin film, then to the orientation of the seed crystals. For example, the MFI synthetic gel which has been used in the secondary growth method usually contains TPAOH. In this case, even when all of the MFI zeolite seed crystals are attached to the substrate such that the a- or b-axes are oriented normal to the substrate, the orientation of the resulting MFI zeolite thin film changes randomly.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present disclosure.